"Of course I won't laugh," the doctor said. "I'm a professional. In over twenty years I've never laughed at a patient."
"Okay then," Bob said, and proceeded to drop his trousers, revealing the tiniest 'whoo-ha' the doctor had ever seen. It couldn't have been bigger than the size of a AAA battery.
Unable to control himself, the doctor started giggling, then fell laughing to the floor. Ten minutes later he was able to struggle to his feet and regain his composure.
"I'm so sorry," said the doctor. "I really am. I don't know what came over me. On my honor as a doctor and a gentleman, I promise it won't happen again. Now, what seems to be the problem?"
"It's swollen," Bob replied.
